The present invention relates to droplet deposition apparatus, particularly inkjet printheads, incorporating piezoelectric actuator elements having first and second portions both deflectable simultaneously to effect ejection of a droplet.
Such printheads are known, for example, from EP-A-0 277 703 (incorporated herein by reference): an ink chamber is bounded on at least one side by an actuator wall constructed from upper and lower portions of piezoelectric material bonded together at their common surface and polarised in the plane of the actuator wall in respective opposite senses. When subjected to an electric field perpendicular to the direction of polarisation by electrodes located on opposite sides of the actuator wall, both upper and lower portions deform in shear towards the ink chamber, thereby causing the ejection of a droplet from the ink chamber via a nozzle. Such an actuator wall construction has acquired the name of xe2x80x9cchevronxe2x80x9d from the shape taken up by the actuator wall as a whole when subject to actuating electric fields.
To date the techniques for manufacturing chevron printheads have been complex, involving either special piezoelectric laminates into which full-depth ink channels are formed (for example WO92/09436) or the intricate alignment of two piezoelectric sheets each formed with half-depth ink channels (for example the aforementioned EP-A-0 277 703). The actuator walls of such known chevron printheads furthermore include discontinuities in the form of adhesive bonds and/or negative temperature coefficient layers.
It is an object of the present invention to avoid the complexities of manufacture and structural discontinuities of known chevron constructions.
Accordingly, the present invention consists of a method of polarising a portion of a monolithic piezoelectric actuator element for use in droplet deposition apparatus, the element having first and second portions, the method comprising the steps of:
placing the first portion of the element at an equipotential, and generating a polarising electric field across the second portion between said first portion and a further conducting layer.
Such a method allows a portion of a monolithic piezoelectric element to be subject to polarising electric fields whilst maintaining another portion of the same element free from repolarisation by holding that portion at an equipotential, advantageously be partial enclosure by a conducting layer which effectively holding that portion free from electric fields in the manner of an at last partial Faraday cage. In the particular case of an actuator element that is initially polarised entirely in one direction, it allows a portion of that element to be repolarised in a opposite direction, the resulting actuator having a chevron poling pattern. The monlithicxe2x80x94i.e. one-piece, integral, homogeneous, uniformxe2x80x94construction of such an actuator avoids the disadvantages of the known constructions discussed above.
A corresponding component for use in droplet deposition apparatus is included in another aspect of the invention and comprises a homogeneous sheet of piezo-material formed with a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels and defined each by facing side walls and a bottom surface extending between the side walls;
at least one of said side walls having a first portion remote from a respective bottom surface and a second portion adjacent said respective bottom surface, said second portion being polarised in a direction normal to both the length of the channels and the array direction and said first portion being either unpolarised or polarised in a direction other than said first direction.
Such a component has side walls polarised according to two different regimes and yet retains a homogeneous structure with the associated strength and ease of manufacture that this implies.
Further advantageous embodiments of the invention are set out in the description and dependent claims.